Nanophotonic Endoscope Gently Probes Single Cells

Photonics.comDec 2011
BERKELEY, Calif., Dec. 21, 2011 — A newly developed nanophotonic endoscope can take high-resolution images of the inside of a single living cell and deliver therapeutic drugs and other cargo without injuring or damaging the cell.
Fluorescence confocal image of a single living HeLa cell shows that, via nanoendoscopy, a quantum dot cluster (red dot) has been delivered to the cytoplasm within the membrane (green) of the cell. (Courtesy of LBNL)
Researchers from Lawrence Berkeley National Laboratory (LBNL) and the University of California, Berkeley, attached a tin oxide nanowire waveguide to the tapered end of an optical fiber to create the endoscope system. Light travelling along the optical fiber can be effectively coupled into the nanowire, where it is re-emitted into free space when it reaches the tip. The nanowire tip is extremely flexible due to its small size and high aspect ratio, yet can endure repeated bending and buckling so that it can be used multiple times.

This schematic depicts the subcellular imaging of quantum dots in a living cell using a nanowire endoscope. (Courtesy of LBNL)
“By combining the advantages of nanowire waveguides and fiber optic fluorescence imaging, we can manipulate light at the nanoscale inside living cells for studying biological processes within single living cells with high spatial and temporal resolution,” said Peidong Yang, a chemist with LBNL’s Materials Sciences Div., who led this research. “We’ve shown that our nanowire-based endoscope can also detect optical signals from subcellular regions and, through light-activated mechanisms, can deliver payloads into cells with spatial and temporal specificity.”

Because cells are optically transparent, they can be noninvasively imaged with visible light in 3-D, and visible light allows the fluorescent tagging and detection of cellular constituents, such as proteins, nucleic acids and lipids. The one drawback to visible-light imaging in biology has been the diffraction barrier, which prevents visible light from resolving structures smaller than half the wavelength of the incident light.

Recent breakthroughs in nanophotonics have made it possible to overcome this barrier and bring subcellular components into view with optical imaging systems. However, such systems are complex, expensive and, oddly enough, bulky in size.
Peidong Yang, a chemist who holds joint appointments with LBNL and UC Berkeley, is a recognized nanoscience authority. (Image: Roy Kaltschmidt, LBNL Public Affairs)
“Previously, we had shown that subwavelength dielectric nanowire waveguides can efficiently shuttle ultraviolet and visible light in air and fluidic media,” Yang said. “By incorporating one of our nanophotonic components into a simple, low-cost bench-top fiber-optical setup, we were able to miniaturize our endoscopic system.”

To test their nanowire endoscope as a local light source for subcellular imaging, the team optically coupled it to an excitation laser, then waveguided blue light across the membrane and into the interiors of individual HeLa cells, the most commonly used immortalized human cell line for scientific research.

“The optical output from the endoscope emission was closely confined to the nanowire tip and thereby offered highly directional and localized illumination,” Yang said. “The insertion of our tin oxide nanowire into the cell cytoplasm did not induce cell death, apoptosis, significant cellular stress or membrane rupture. Moreover, illuminating the intracellular environment of HeLa cells with blue light using the nanoprobe did not harm the cells because the illumination volume was so small — down to the picoliter scale.”

Images of a nanowire endoscope in close contact with a quantum dot cluster in a HeLa cell (left), and separated vertically from the cluster by 2 mm (middle) and horizontally by 6 mm (right). Colored circles and arrows mark the position of the cluster and movement of the endoscope. (Courtesy of LBNL)
Having demonstrated the biocompatibility of their nanowire endoscope, Yang then tested its capabilities for delivering payloads to specific sites inside a cell. While carbon and boron nitride nanotube-based single-cell delivery systems have been reported, these systems suffer from delivery times that range from 20 to 30 minutes, plus a lack of temporal control over the delivery process. To overcome these limitations, the team attached quantum dots to the tin oxide nanowire tip of their endoscope using photoactivated linkers that can be cleaved by low-power ultraviolet radiation. Within one minute, their functionalized nanowire endoscope was able to release its quantum dot cargo into the targeted intracellular sites.

“Confocal microscopy scanning of the cell confirmed that the quantum dots were successfully delivered past the fluorescently labeled membrane and into the cytoplasm,” Yang said. “Photoactivation to release the dots had no significant effect on cell viability.

“In the future, in addition to optical imaging and cargo delivery, we could also use this nanowire endoscope to electrically or optically stimulate a living cell,” Yang said.

A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.

Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.